Short wave reception



March M, 1.941.l B TREVOR ETAL SHORT WAVE RECEPTION' original Filed Dec.e, 1935 Patented Mar. 11, 1941 UNITED STATES PATENT OFFICE SHORTWAvEfREcEPTIoN original application December 6, 1935, serial No.

53,136. Divided and this 19, 1938, Serial No. 185,722

application January 1c claims. (o1. 17e-.44)

The present invention is a division of our copending application SerialNo. 53,136, iiled December l6, 1935, now United States Patent'No."2,152,335, granted March 28, 1939, and relates to improvements in bandpass transformers or frequency selective circuits.

An object of the invention is to provide an Y improved signal selectingcircuit for use at very short waves, so as to properly feed the desiredvsignal into the first detecton'grid, orinput circuit. A suitableselecting circuit consists, according to our present invention, ofa'metallic rod, pipe or'tube grounded at one end andn adjusted in lengthso as to resonate at the desired incoming carrier frequency. The antennacan be connected to one point'along this rod and the gridof 'the firstdetector to another point along the rod, so chosen 'as to suitably loadthe rod and at the same time derive from the road or tube a suitableexcitation voltage for the `lirst detector. Such an arrangement,however,- in general would. suffer from the disadvantage of 'being tooselective and being rather diiiicult to adjust.4

Toovercome these objections is a still further object of our presentinvention, and this 'object we accomplish by providing a tuningcondenser at the Vfar end, that is atthe ungrounded endof the rod, so asto permit ease of tuning. To broaden the frequency pass band'of therod',resistance is added to the" circuit in a way which is described morefully hereinafter. In" addition, we find it advisable to shield the yrodortubeby adding a grounded metalliccylinder around the tube.

This single rod arrangement, while it o'ifers much that is to bedesired, still lacks; in certain instances, the ability to pass asuii'iciently Wide band of frequencies into the firstdetectonparticularly when the short Wave receiver is used, for example,for facsimile work.V Accordingly, a, 'further object of oui` presentinvention is to provide an improved tube or rod circuit making use of amultiplicity-of tubes or rods, preferably having overlapping resonancecharacteristics Such that, in toto, the group of rods is connected toserve as a rband pass filter passing the desired bandof frequencies intothe iirst detector.

Other objects and features will appear from a reading of the followingdetailed description-of the invention in conjunction with a drawingWhose single figure illustrates, schematically, the selective circuit ofthe invention in 'an improved ultra short wave system. The systemillustrated in the drawing is given by wayof example only,

' associated metallic cylinders and is not to be construed as limitingthe application of the invention to other systems or circuits. iReferring to the drawing, high frequency energy is picked up on anysuitable antenna, such as the diamond antenna 2, and fed to the primary4 of the transformer variably coupled to the secondary 6. The twowindings are electrostatically screened from each other by a screendiagrammatically indicated Vat 8 and vmounted within a metallic cylinderI0,`in turn facing into and electrically and physically connected to thecylinder I2. Concentrically mounted within the cylinder I2 therev is arod, tube, or metal pipe I4, grounded at its left-hand end I6 andconnected Vto ground at its right-hand end Vthrough a damping resistor-I8 and variable tuning condenser 20, the combination forming anon-radiating highly selectivecircuit. The metal-rod I4 within themetalcylinder I2" forms a' sharply selective inlput circuit, standingcurrent waves being set up thereon by virtue o'f the input from thesecondary 6 and by virtue of the tuning of condenser 20. In order towiden the band Width of this sharply selective prcselector circuit, thedamping resistance I8 has been provided.

The received energy resonated in the rod I4 and condenser 20 is fedthrough a low impedance connection 22 to a second rod, tube or metallicpipe 24 mounted within a second cylinder 26 which is grounded, as shown.'I'he far end of the second rod 24 is tuned by means of the variabletuning condenser 28. The rod 2'4 and condenser 28 form a secondnon-radiating highly selective circuit and are adjusted to have aresonance curve which overlaps the resonance curveof A'the rodI4-condenser 20 combination. In this way the vfrequency pass band widthof the two preselector circuits I4, 20, I2 and 24,

28, 26`is further widened.

That is to say, the two rods I4, 24 with their I2 and 26, thedamping'resistor I8, and the tuning condensers 20, 28 form a preselectorlter for our improved ultra-short wavelength radio receiver. Couplingbetweenthe ,tuned circuits of our improved filter is adjusted byvaryingthe taps of the low im- .pedance connection 22 along the rods I4, 24. It

is to be noted-that atthese high frequencies the grid of the detectortube presents a resistance, .load to rod 24 which forms part of thedamping of the transformer systeml I2, I4, 24, 26, I8, 20,

The received filtered energy is fed through lead 30 onto the grid 32 ofthe high-frequency detector tube 34 which, in addition, is provided witha heating filament 36, a heater surface or cathode 38 and a plate oranode 40. Local highfrequency oscillatory energy is injected into thedetector tube 34 through the cathode leads, as will be explained morefully hereinafter, and the beat-frequency energy is resonated irl theplate circuit of the rst detector 34 by means of the tuning coil 42 andthe tuning condenser 44, both of which are adjusted to the beatfrequency. The beat frequency energy is fed through the bypassingcondenser 46 and intermediate-frequency energy leadl A to theintermediate-frequency amplifer.

The rst local oscillation generator is provided with a vacuum tube 48having a grid 50, a cathode 52, 'a heating filament 54 and a plate oranode 56. The platecircuit of the tube 48 is provided with a platetuning coil 58, a plate tuning condenser 60, a by-passing condenser 62.Plate voltage is `fedthrough the lead 64 which is kept at groundlrado-'frequency potential by the action of the lbypassing condenser62`, and also the resistance con- --denser lters 66, 68 and 10, 12,housed within the ashieldingcompartment 14, 16. Within the shield 14',16 there are also provided the coil and conldenser'filters 18, 80 and82, 84, which, together with the by-passing condensers 86, closelyadjacent `to the heater filament 54, maintain the heater at groundradio-frequency potential.

`Proper grid bias .is maintained by the action of `tl'iecondenser andgrid-leak combination 8l'.

oscillations are generated because of plate-to- Vgrid interelectrodefeed-back, and the frequency of oscillations is maintained constant byvirtue of the improved frequency-controlling grid circuit, which willnow be described in detail The radio-freqnency grid tank circuit rforthe oscillator 48 is formed of a cast metal cylinder 90 having agrounded base portion 92. From the vbase portion and integral therewithythere extends the central projection 94, whichin turn is capped with acast metal disc 96. Screwed into the open end of the cast metal cylinder90, opposite the base 92, is another cast metal disc 99. vThis structureforms a high-frequency'tuned circuit of eX- ceptionally good mechanicalvrigidity and is made of metal of low temperature coefficient, such asInvar, and will maintain its electrical characteristics constant over awide range of temperature. hence insuring constancy of 'frequency ofoper- -ation of the first oscillator 48.

In general, it may bevsaid that the confronting surfaces of the disc 96and the screw threaded disc-98 form a low-loss condenser, and the lengthof the metallic projection between the disc 96 and `the base 92, alow-loss inductor. This grid circuit is non-radiating, since the linesof flux are 'all contained within the cylinder 90, the base 92 and thedisc 98. These latter elements, namely the outer surfaces of the base92, cylinder and Vdisc 98, are maintained at ground radio-frequencypotential, and can be directly grounded, if desired, for direct currentsas well.

From another viewpoint it may be said that vhigh-frequency currentssurge back and forth along the length of direction `of the outercylindrical surface of vthe projection 94, as a consequence of which themagnetic 'flux lines travel circularly in thespace between theprojection 94 and the cylinder 90. As the high-frequency currents flowback and forth at an exceedingly high rate, the flux lines build up andcollapse through 'the cathode conductor |00 connected to the cathode 38of the detector tube 34, and `also through the heating leads |02 for theheater 36 of the first detector 34.

The frequency of the local oscillator is adjusted by screwing the metaldisc 98 towards or away from the metallic disc 96. A micrometer 5 gauge|04, bearing at one side and supported by the fixed supports |06, and atits ,other-movable side |08 upon the disc 98, serves as atuningindicator indicating the frequency of oscillations generated by thefirst local oscillation generator 48. l0

It Vhas already been pointed out that -the grid circuit for the localoscillator may be made of cast metal, such as copper, or of Invar withcopper or silver plating after machining to size, so as to give alow-temperature coefficient of frequency 15 variation. In addition, orin the alternative, the entire oscillator may be placed in a temperaturecontrolled oven, and/or placed in a room which is roughlytemperature-controlled to still further enhance frequency stability.

Because of the lines of magnetic flux which build up and collapsethrough the cathode conductor |00 for the Vfirst detector 34, thecathode 38 of the detector vibrates in potential at the frequency of theoscillations produced by the local y25 oscillator 48. In addition,thegrid 32 of the rst detector 34 is vibrated through lead 30, as beforeexplained, in potential and at the frequency of the incoming wave whichmay be of the order'of 100 megacycles or three Imeters in Wavelength. 30VIthas been foundpreferable to operate the local oscillator 48 abovetheincomlng wave frequency to produce in the beat-frequency circuit 42,44, and also in the beat-frequency lead A, beat-frequency energy of-theorder of 28 megacycles. It 35 is yto be clearly understood, however,thatthese -gures are given simply by way of example, and `are not to beconstrued ln anyway as-1imitingthe "present invention to'these specificfrequencies or wavelengths. A40 As was done with the case ofthelocaloscillator 48, the first detector plate circuit is providedwith agrounding condenser ||0 and resistive and capacitive lters ||2, ||4within the shielding compartments ||6, I8. Also, the vheating leads |02l5 for the first detector filament 3,6, are provided vwith-the chokecoil and condenser'lters |2`0, |24,

-circuit forthe first detector 34, which tank circuit consists of thetuning coil 42, the tuningi60 condenser` 44 and a resistance 45broadening the frequency pass band of a plate circuit of the firstdetector 34.' The beat-frequency energy is fed through the lead A|28 toa suitable utilization circuit, such as an intermediate frequencyamplifier, not shown.

The metal cylinders |2, 26 of the frequency selective circuit arepreferably rigidly mounted 'against the base of the radio frequencytuned F circuit 90, 94, 96, 98 to provide for 'additlonalio mechanicaland electrical stability. A housing is bolted against the-'base 90 andcontains such elementsfas resistance |8 and condensers 20 and v28.Hollows maybe provided in the-casting 90,

92 so that the tubes 48 and 34, respectively the 7-5 Y of Xed or Zeroalternating current potential.

What is claimed is:

1. In combination, a high frequency circuit comprising a metallic rod, aconnection having very low impedance'to energy of the operatingfrequency from ground to a point on said rod, a variable condenserconnecting another point on said rod to ground, and a resistanceconnected in shunt to a portion of said rod and ground, an `inputcircuit coupled to one portion of said inner conductor, and an outputcircuit coupled to another portion of said inner conductor.

2. In combination, a high frequency selective circuit comprising aninner conductor and a surrounding outer conductor directly connectedtogether at one point in their lengths, a connection having very lowimpedance to energy of the o-perating frequency from said one point toground, a variable condenser connecting said inner and outer conductorstogether at another point in their lengths, and a resistor connectedacross said inner and outer conductors at alocation removed from saidfirst point, an input circuit coupled to said inner conductor at oneplace in its length, and an output circuit coupled to said innerconductor at another place in its length.

3. A lter circuit comprising a pair of linear conductors havingoverlapping resonance curves, an input circuit coupled between twopoints on one of said conductors, an output circuit connected to theother of said -conductors, and a connection devoid of concentratedreactance coupling said conductors together at points intermediate theirends.

4. Apparatus as lclaimed in claim 3, characterized by the fact that agrounded metallic shield is placed about each 'of said linear metallicconductors.

5. A filter comprising a linear metallic conductor grounded at one end,a grounded metallic shield about said conductor, a variable condenserconnected from the free end of said conductor to ground, a resistanceconnected in shunt to a portion of said conductor, a second linear conductor also grounded at one end, a grounded metallic shield about saidsecond conductor, said second linear conductor cooperating with itsshield to form a tuned circuit, and means for coupling said linearconductors together.

6. In combination, a tuned circuit comprising an inner andan outerconductor conductively coupled together at one end, another tunedcircuit also comprising an inner and an outer conductor conductivelycoupled together at one end,

a connection from a point on one inner conductor intermediate its endsto a point on said other inner conductor intermediate its ends, an inputcircuit coupled to the conductors of only one of said tuned circuits,and an output circuit coupled to the conductors of the other tunedcircuit.

7. A frequency selective circuit having, in combination, a tuned circuitcomprising an inner and an outer conductor conductively coupled togetherat one end, another tuned circuit also comprising an inner and an outerconductor coupled togetherxat onefend, a connection from a point onone'inner conductor intermediate its ends toa point on said other innerconductor intermediate its ends, means for maintaining said outerconductors of said tuned circuits at a rela'- tively fixed radiofrequency potential, an input circuit coupled to the conductors of onlyone of said tuned circuits, and an output circuit ycoupled to theconductors 'of the other'tuned 8. A frequency selective circuit having,in c ombination, a tuned circuit comprising van inner and an outerconductor conductively coupled together at one end, `an impedanceconnecting the other ends of said conductors together; an- 15 othertuned circuit also comprising an inner and an outer conductor coupledtogether at one end, an impedancealso co-nnecting the other ends of thislast tuned circuittogether; a direct connection between said innerconductors of said tuned circuits; an input circuit coupled to theconductors of one tuned circuit; and an output circuit coupled to theconductors of the other tuned circuit. t l.

9. The combination with a frequency selective circuit having a tunedcircuit comprising an inner and an outer conductor conductively coupledtogether at one end, and another tuned 'circuit also comprising an innerand an outer conductor coupled together at one end, means formaintaining said outer lconductors at a fixed radio frequency potential,a direct connection from a point on one inner conductor intermediate itsends to a point on the other inner conductor intermediate its ends, ofan input circuit coupled to only one of said inner conductors at anotherpoint intermediate its ends, and an output circuit coupled to the otherof said inner conductors.

10. A coupling circuit between a source of high frequency oscillationsand ya load circuit, cornprising two frequency selective circuits eachbeing a section of a coaxial line, the inner conductors of said linesbeing coupled together at locations intermediate their ends, animpedance connected in shunt to the conductors of one frequencyselective concentric line for widening the pass band thereof, saidsource being coupled to one of said lines, and said load being coupledto the other of said lines.

11. A coupling circuit between -a source of high frequency oscillationsand a load, comprising two frequency selective circuits` each being inthe form of a concentric line having an inner and an outer conductorconductively coupled together at one end, a capacitive connection from apoint on each inner conductor near its other end to its associated outerconductor, a damping resistor connecting one of said inner conductors toits associated outer conductor also near said last end for widening thefrequency band Width of that frequency selective circuit of which itforms a part, and means coupling together the inner conductors of saidtwo concentric lines.

12. A system in accordance with claim 10, characterized in this thatsaid source of high frequency oscillations is coupled to the innerconductor of one of said concentric lines near the conductively coupledend thereof, and said load is coupled to the inner conductor of theother concentric line near the end removed from the conductively coupledend.

13. The combination with a frequency selective circuit having `a firsttuned circuit comprising an inner and an outer conductor conductivelylcoupled together at one end and capacitively rcoupled togetherrat theother end, and a second similarly arranged tuned circuit, one of saidtuned circuits having means at the capacitively coupled end forbroadening the frequency response characteristic thereof, a directconnection between points on said inner conductors intermediate the endsthereof, of an input circuit coupled to one of said inner conductors andan output circuit coupled to the other one of said inner conductors.

14. A lter circuit comprising a pair of linear conductors havingoverlapping resonance curves, means for widening the frequency responsecharacteristic of one of said conductors, an input circuit coupledbetween two points on one of said conductors, an output circuitconnected to the other of said conductors, and a connection devoid ofconcentrated reactance coupling said conductors together rat pointsintermediate their ends.

15. A band .pass selective circuit comprising a tuned circuithaving aninner and an outer conductor conductively coupled together at one oftheir adjacent ends and capacitively coupled to gether at their otheradjacent ends, another similar tuned circuit having a resonancecharacteristic overlapping the resonance characteristic of the rst tunedcircuit, a resistance connected being a resonance characteristicoverlapping the resonance characteristic of the rst tuned circuit, aconnection from each of said outer conductors to a point of relativelyfixed radio frequency potential, a resistance connected between theinner and outer conductors of one of said tuned circuits for broadeningthe frequency pass band thereof, a connection between the innerconductors of said tuned circuits, an input circuit coupled to one tunedcircuit and an output circuit coupled to the other tuned circuit.

BERTRAM TREVOR. RALPH W. GEORGE.

